1. Field of the Invention:
This invention relates to an apparatus for the detection of the overheating of electrical insulation and to gas cooled dynamoelectric machine systems incorporating such apparatus.
2. Description of the Prior Art:
During recent years, a device commonly known as a generator condition monitor (Ref: U.S. Pat. No. 3,427,880 issued Feb. 18, 1969 and U.S. Pat. No. 3,573,460 issued Apr. 6, 1971) has been used to detect overheating within a dynamoelectric machine, such as overheated electrical insulation, through the presence of thermoparticulates in the dynamoelectric machine's cooling gas. Thermoparticulates are formed in dynamoelectric machines as a product of thermal degradation of insulation. These particulates have radii of the order of between 1 .times. 10.sup.-9 and 100 .times. 10.sup.-9 meters. In the monitor, the molecules of a steadily flowing sample of the cooling gas are ionized to a state of equilibrium with a source of alpha-particles in an ionization chamber. The ions are then completely electrodeposited when the gas is passed between two charged electrodes in a collection chamber. The electrodeposition current is then amplified and applied to a recorder where it is continuously monitored. If there have been thermoparticulates entrained within a given sample, some of the ions will attach themselves to the thermoparticulates causing the number of free ions to decrease. The charged thermoparticulates have a much lower mobility as compared to the cooling gas ions; therefore, very few will be electrodeposited resulting in a decrease in the total electrodeposition current. This decrease is used as an indication of the presence of thermoparticulates caused by overheating.
A weakness of the generator condition monitor as described above and as it presently is used is that changes in the gas pressure, gas purity, or the flow through the monitor, as well as contamination of the radioactive source, can also cause a descrease of the electrodeposition current, thus falsely indicating a condition of overheating.
In copending Application Ser. No. 732,636, filed Oct. 15, 1976, by Dillman and assigned to the assignee of the present invention, now abandoned, there is disclosed an improved generator condition monitor. The Dillman invention generally comprises two streams which have been extracted from the cooling gas circulated through a generator and two thermoparticulate detectors operating in parallel with one detector monitoring one gas stream and the other detector monitoring the second gas stream after it has been filtered. Variation between the output signals of the detectors yields an overtemperature alarm which is independent of changes in the parameters of the cooling gas streams being monitored.
However, the Dillman invention requires servicing of the filters and manual intervention for verification of an alarm by inserting a filter in the unfiltered stream. If the alarm does not cease when the second filter is inserted, then the indication is that the alarm was false and one of the ionization chambers' radiation source is ineffective.